The present invention relates to the chemical modification of polyimide membranes. In particular it relates to the chemical modification of polyimide membranes which form one layer (preferably the outer-layer) of dual-layer hollow fibres.
Polyimides are attractive membrane materials for gas separations because of their good gas separation and physical properties. Extensive work, including tailoring the chemical structures and performing cross-linking modifications by different methods such as thermal treatment, chemical treatment and UV irradiation, has been carried out aimed at obtaining polyimide membranes with better gas separation properties. Among all these efforts, cross-linking modification is expected to be the most promising approach to obtain better membranes which can be used under complex and harsh environments because it can impart polyimide membranes with anti-plasticization properties and improved chemical resistance.
Most commercial membranes are in the form of hollow fibres because they offer a higher surface to volume ratio. Each hollow fiber membrane usually has an asymmetric cross-section morphology which consists of a thin dense selective layer and a porous supporting substrate. The asymmetric morphology gives the advantage of high flux which is required for practical applications of such membranes. U.S. Pat. No. 5,085,676 discloses a process for preparing dual-layer hollow fibre gas separation membranes which structurally consist of a thin selective layer (usually the outer-layer) and a porous supporting substrate (usually the inner-layer). Not only do these dual-layer membranes have high flux advantages as for other asymmetric membranes, they also optimise materials performance and reduce materials costs.
However, most polyimides suffer plasticization or chemical attack induced by the sorption of CO2, H2S or other chemicals. Almost all the reported cross-linking modifications of polyimides to enhance chemical resistance and anti-plasticization characteristics have been conducted on thick and flat dense films, which may have very limited applications for the modification of hollow fibre membranes. For example, U.S. Pat. No. 4,717,393 presents photo-chemical methods for the cross-linking modification of particular polyimides containing benzophenone groups and hydrogen donor groups such as methyl groups. Although this method produces cross-linked polyimides with high gas permselectivity, the gas permeability of these cross-linked polyimides is too low. U.S. Pat. No. 4,981,497 describes a process to modify polyimide membranes with amino compounds. The modification results in lower gas permeation rates as compared to the uncross-linked membranes but is limited to thick dense polyimide films and requires thermal treatment in order to complete the reaction. U.S. Pat. No. 4,931,182 discloses a class of polyimide membranes containing copolymerizable, surface-modifiable units containing both aromatic diamines and alkenylated diamines having a vinyl or vinylaryl group preferably positioned ortho to an amine functionality. The polyimide membranes can be cross-linked by treatment with an activating force such as high energy electromagnetic irradiation or with a free radical source to impart high selectivity to the membranes with a large decrease in composite permeance. Unfortunately, hollow fibres from these kinds of polyimides cannot be easily fabricated.
Therefore, it is essential to investigate new and practical cross-linking modification technologies for polyimide membranes, in particular with the aim of finding processes which can be suitably applied to the manufacture of hollow fibres.
Accordingly, the present invention provides a process for chemically modifying a dual-layer hollow fibre, wherein said fibre comprises a first layer consisting essentially of a polyimide and a second layer consisting essentially of a polymer which is substantially unaffected by the chemical modification process, which process comprises contacting said polyimide layer with a polyamine.
As used herein the term xe2x80x9cpolyimidexe2x80x9d includes blends of two or more different polyimides.
As used herein the term xe2x80x9cpolymerxe2x80x9d includes copolymers and blends of two or more different polymers and/or copolymers.
As used herein, the term xe2x80x9csubstantially unaffected by the chemical modification processxe2x80x9d when used to qualify the nature of the polymer, means that the physical and/or chemical properties of the polymer which make it suitable for use as a support layer for the polyimide remain unaffected by the chemical modification process or are only affected to an extent which does not significantly affect its performance as a support layer for the polyimide.
In a preferred embodiment, the polyimide forms the outer-layer of the dual-layer hollow fibre and the polymer which is substantially unaffected by the chemical modification process forms the inner-layer of the dual-layer hollow fibre.
In one embodiment of the invention, the polyamine may contact only one side of the polyimide layer.
Preferably, the polyimide layer is contacted with the polyamine at a temperature in the range of from 5xc2x0 C. to 50xc2x0 C., more preferably in the range of from 15xc2x0 C. to 30xc2x0 C.
In one embodiment of the invention, the polyamine is contacted with the polyimide layer in the form of a solution in a suitable solvent. In this case, contact may be effected by simply dipping the dual-layer hollow fibre in the solution. When a solution of polyamine is used the polyimide layer is preferably washed with the solvent after contact with the polyamine solution. Preferred solvents include water and alcohols which are liquid at ambient temperature, such as methanol. Methanol is particularly preferred.
In a preferred embodiment, at the end of the process, the dual-layer hollow fibre is dried at a temperature in the range of from 5xc2x0 C. to 80xc2x0 C., more preferably at a temperature in the range of from 15xc2x0 C. to 40xc2x0 C.
Whilst the process of the present invention could be applied to many types of polyimide, a preferred polyimide for use in the polyimide layer is an aromatic polyimide membrane.
Preferably the polyimide layer consists essentially of the following structural units: 
where each of the n Ar1 groups is a quadrivalent aromatic moiety independently selected from the group consisting of: 
and where each of the n Ar2 groups is a bivalent aromatic moiety independently selected from the group consisting of: 
where Z is selected from the group consisting of: 
and where X, X1, X2 and X3 are each independently selected from hydrogen, C1-5alkyl, C1-5alkoxy, phenyl or phenoxy.
The value of n must be sufficient to provide a viable polymer membrane for use as the polyimide layer of a dual layer hollow fibre. Preferably, n is a number sufficient that said polymer has an inherent viscosity of at least 0.3 dL/g as measured at 25xc2x0 C. on a 0.5% by weight solution in N-methylpyrrolidinone.
In a preferred embodiment of the present invention, the polyamine is an aliphatic-aromatic polyamine. More preferably, the polyamine is an aliphatic-aromatic diamine. Even more preferably, the polyamine is an aliphatic-aromatic diamine having the general structure:
H2N(CH2)axe2x80x94Ar2xe2x80x94(CH2)bNH2
where Ar2 is as defined above and a and b are each independently selected from the range 1 to 6. Still more preferably, the polyamine is an aliphatic-aromatic diamine having the general structure: 
Most preferably, the polyamine is selected from m-xylylenediamine or p-xylylenediamine.
As previously mentioned, the polyamine may be used in the form of a solution. In a preferred embodiment of the process, the polyamine is contacted with the polyimide layer in the form of a solution having a concentration of polyamine of from 2 wt % to 50 wt % based on the total weight of the solution. More preferably, the polyamine is contacted with the polyimide layer in the form of a solution having a concentration of polyamine of from 2 wt % to 20 wt % based on the total weight of the solution. Most preferably, the polyamine is contacted with the polyimide layer in the form of a solution having a concentration of polyamine of from 5 wt % to 10 wt % based on the total weight of the solution.
The identity of the inert (or non-crosslinked) layer material is not critical to the present invention. It is simply necessary that it remain substantially unaffected by the modification process. Suitable polymers for forming the inert layer may be selected from polyesters, polysulfone, polycarbonate, polyetherketone, polyetheretherketone, polyethersulfone, polyetherimide, polyacrylonitrile, polyamide and polyolefin. Polyethersulfone and polysulfone are particularly preferred.
The present invention also provides for a chemically modified dual-layer hollow fibre obtainable by a process as previously described.
The present invention also provides for a process for separating at least one gas from a mixture of two or more gases, which process comprises bringing a pressurised mixture of said gases into contact with a dual-layer hollow fibre obtainable by a process as previously described. The gas mixtures for separation may be selected from mixtures of He and N2, H2 and N2, O2 and N2, CO2 and CH4, CO2 and H2S and CH4, and CO2 and H2S and CH4 and C2-C4 hydrocarbons. The dual-layer hollow fibre obtainable by a process as previously described is particularly useful for separation of a mixture of CO2 and CH4 or a mixture of CO2 and H2S and CH4.
In another aspect, the present invention also provides a process for chemically modifying a polyimide membrane, which process comprises contacting said membrane with an alcoholic solution of an aliphatic-aromatic polyamine.
Alcohols suitable for use as the solvent include any alcohol which is liquid at ambient temperature and is capable of solubilising the aliphatic-aromatic polyamine. Methanol is a particularly preferred solvent.
Preferably, the membrane is contacted with the solution at a temperature in the range of from 5xc2x0 C. to 50xc2x0 C., more preferably at a temperature in the range of from 15xc2x0 C. to 30xc2x0 C.
In a preferred embodiment of the process, the membrane is washed with the alcoholic solvent after contact with the solution.
In another preferred embodiment of the process, after contact with the solution or after being washed with the alcoholic solvent, the membrane is dried at a temperature in the range of from 5xc2x0 C. to 80xc2x0 C., more preferably at a temperature in the range of from 15xc2x0 C. to 40xc2x0 C.
In a preferred embodiment of the process, the polyimide membrane is an aromatic polyimide membrane. More preferably, the polyimide membrane consists essentially of the following structural units: 
where each of the n Ar1 is a quadrivalent aromatic moiety independently selected from the group consisting of: 
and where each of the n Ar2 groups is a bivalent aromatic moiety independently selected from the group consisting of: 
where Z is selected from the group consisting of: 
and where X, X1, X2 and X3 are each independently selected from hydrogen, C1-5alkyl, C1-5alkoxy, phenyl or phenoxy.
The value of n must be sufficient to provide a viable polymer membrane. Preferably, n is a number sufficient that said polymer has an inherent viscosity of at least 0.3 dL/g as measured at 25xc2x0 C. on a 0.5% by weight solution in N-methylpyrrolidinone
In a preferred embodiment of the process, the aliphatic-aromatic polyamine is an aliphatic-aromatic diamine. More preferably, the aliphatic-aromatic polyamine is an aliphatic-aromatic diamine having the general structure:
H2N(CH2)axe2x80x94Ar2xe2x80x94(CH2)bNH2
where Ar2 is as defined above and a and b are each independently selected from the range 1 to 6. Still more preferably, the aliphatic-aromatic polyamine is an aliphatic-aromatic diamine having the general structure: 
Most preferably, the aliphatic-aromatic polyamine is selected from m-xylylenediamine or p-xylylenediamine.
Preferably, the aliphatic-aromatic polyamine is present in the solution in a concentration of from 2 wt % to 50 wt % based on the total weight of the solution. More preferably the aliphatic-aromatic polyamine is present in the solution in a concentration of from 2 wt % to 20 wt % based on the total weight of the solution. Most preferably, the aliphatic-aromatic polyamine is present in the solution in a concentration of from 5 wt % to 10 wt % based on the total weight of the solution.
In a preferred embodiment, the polyimide membrane is in the form of a hollow fibre.
In a particularly preferred embodiment, the polyimide membrane is the selective layer of a dual-layer hollow fibre, the support layer of said dual-layer hollow fibre comprising a polymer which is substantially unaffected by the chemical modification process. Most preferably, the polyimide membrane forms the outer layer of said dual-layer hollow fibre.
The novelty of the present invention resides in the use of alcoholic (preferably methanolic) solutions of rigid aliphatic-aromatic diamines such as p-xylene diamines as a chemical cross-linking reagent for polyimides and in particular for dual-layer hollow fibres with polyimides as the selective layer.
Advantageously, the modification can be performed at ambient temperature, and adopting inert materials as the support layer of dual-layer hollow fibre membranes helps to reduce the side effects of chemical modification which often makes such processes unsuitable for application to dual-layer hollow fibres. This invention provides a feasible method for the production of high-selectivity and anti-plasticisation dual-layer hollow fibres for gas separation and also provides an improved general method of chemical modification of polyimides (which may be in the form of single or dual-layer hollow fibres for gas separation).
The processes of this invention can produce high-selectivity and anti-plasticisation single-layer and dual-layer hollow fibres containing a chemically cross-linked polyimide layer for gas separation. The gas mixtures include the mixtures of H2/N2, CO2/CH4 and O2/N2. Also the membranes fabricated can be used for separation of volatile organic mixtures.